The present invention relates to an ink jet head with which ink droplets are ejected from ejection ports formed in a substrate toward a recording medium, and an ink jet recording apparatus using the ink jet head.
An ink jet recording apparatus ejects ink through ejection ports to record an image corresponding to image data on a recording medium. Examples of known ink jet recording apparatuses include an electrostatic type, thermal type, and piezoelectric type ink jet recording apparatuses which are classified depending on differences of means for controlling ejection of ink.
Hereinafter, the electrostatic ink jet recording apparatus will be described as an example. In the electrostatic ink jet recording apparatus, ink containing charged colorant particles (charged color particles) is used, and predetermined voltages are respectively applied to ejection portions of an ink jet head in accordance with image data, whereby ejection of the ink from the ink jet head is controlled by utilizing electrostatic forces to record an image corresponding to the image data on a recording medium. Known as an example of the electrostatic ink jet recording apparatus is an ink jet recording apparatus disclosed in JP 10-138493 A.
FIG. 8 is a schematic view showing a construction of an example of an ink jet head of the electrostatic ink jet recording apparatus disclosed in JP 10-138493 A. In an ink jet head 100 shown in FIG. 8, only one ejection portion of the ink jet head disclosed in JP 10-138493 A is conceptually shown. The ink jet head 100 includes a head substrate 102, an ink guide 104, an insulating substrate 106, a control electrode 108, a counter electrode 110, a D.C. bias voltage source 112, and a pulse voltage source 114.
Here, the ink guide 104 is disposed on the head substrate 102, and a through hole (ejection port) 116 is bored through the insulating substrate 106 so as to correspond in position to the ink guide 104. The ink guide 104 extends through the through hole 116, and its projecting tip portion 104a projects upwardly and beyond a surface of the insulating substrate 106 on a side of a recording medium P. In addition, the head substrate 102 is disposed at a predetermined distance from the insulating substrate 106. Thus, a passage 118 of ink Q is defined between the head substrate 102 and the insulating substrate 106.
The control electrode 108 is provided in a ring-like shape on the surface of the insulating substrate 106 on the side of the recording medium P so as to surround the through hole 116 of every ejection portion. In addition, the control electrode 108 is connected to the pulse voltage source 114 for generating a pulse voltage in accordance with image data. The pulse voltage source 114 is grounded through the D.C. bias voltage source 112.
In addition, the counter electrode 110 is disposed at a predetermined distance from the ink guide 104 so as to face the tip portion 104a of the ink guide 104 and is grounded. The recording medium P is disposed on a surface of the counter electrode 110 on a side of the ink guide 104. That is to say, the counter electrode 110 functions as a platen for supporting the recording medium P.
During the recording, the ink Q containing colorant particles which are charged in the same polarity as that of a voltage applied to the control electrode 108 is circulated through the ink passage 118 from the right-hand side to the left-hand side in FIG. 8 by a circulation mechanism for ink (not shown). In addition, a high voltage of 1.5 kV for example is continuously applied to the control electrode 108 by the D.C. bias voltage source 112. At this time, the Coulomb attraction between the bias voltage applied to the counter electrode 110 and the electric charges of the colorant particles in the ink, the viscosity of the ink (dispersion medium), the surface tension, the repulsion among the charged particles, the fluid pressure when the ink is supplied, and the like operate in conjunction with one another. Thus, the balance is kept in a meniscus shape as shown in FIG. 8 in which the ink slightly rises from the ejection port (nozzle) 116.
In addition, the colorant particles migrate to move to the meniscus surface due to the Coulomb attraction or the like. In other words, the ink Q is concentrated on the meniscus surface.
If a pulse voltage of for example 0 V is applied from the pulse voltage source 114 to the control electrode 108 biased at 1.5 kV by the bias voltage source 112, then a voltage of 1.5 kV obtained by superposing both the voltages on each other is applied to the control electrode 108. In this state, an electric field strength in the vicinity of the tip portion 104a of the ink guide 104 is relatively low, and hence the ink Q that contains the colorant particles concentrated at the tip portion 104a of the ink guide 104 does not fly out from the tip portion 104a of the ink guide 104.
On the other hand, if a pulse voltage of for example 500 V is applied from the pulse voltage source 114 to the control electrode 108 biased at 1.5 kV, then a voltage of 2 kV obtained by superposing both the voltages on each other is applied to the control electrode 108. As a result, the ink Q containing the colorant particles which are concentrated at the tip portion 104a of the ink guide 104 flies out in the form of ink droplets R from the tip portion 104a of the ink guide 104 by the electrostatic force, is electrostatically attracted by the grounded counter electrode 110 and adheres to the recording medium P to form thereon a dot of the colorant particles.
In such a manner, recording is carried out with the dots of the colorant particles while the ink jet head 100 and the recording medium P supported on the counter electrode 110 are relatively moved to thereby record an image corresponding to the image data on the recording medium P.
Here, in the image recording with the ink jet head for ejecting ink droplets from the ejection ports, the meniscus needs to be stably formed in order to stably eject the ink droplets.
However, in the case of the ink jet recording apparatus disclosed in JP 10-138493 A, since the holding property of the formed meniscus is poor and thus the meniscus shape is not stabilized, the ejection performance fluctuates. For this reason, there is encountered a problem in that the drawing cannot be satisfactorily carried out.
In addition, when the holding property of the meniscus is poor, the formed meniscus is broken, and thus the ink overflows from the ejection port. As a result, there is also encountered a problem in that since the surface of the ejection port substrate gets dirty, the cleaning and maintenance for the surface of the ejection port substrate need to be carried out.